Steam turbines in current generation, in combined-cycle operation and in the chemical industry are expected to have a high degree of availability. If changes to a steam turbine occur which reduce efficiency and, if appropriate, cause a shutdown, this leads to high outage and consequential costs. An early diagnosis of imminent changes to the machine parts of a steam turbine allows conditioned-oriented maintenance planning and thus reduces the operating costs.
An essential information source for assessing the availability and viability of a steam turbine is the knowledge of the condition of those components of the steam turbine around which or through which steam flows during operation. Thus, operators fear, for example, deposits in steam turbines, since these, in addition to reducing the power output and efficiency, may entail an overloading of individual components which is harmful to the plant.
Depending on the type of construction and the field of use, every steam turbine, as a system, exhibits a typical thermodynamic behavior. If the thermodynamic behavior of a steam turbine changes due to faults occurring on components around which steam flows, it is appropriate to detect these changes in relation to normal behavior, so that damage avoidance or at least damage limitation can be put into effect at an early stage. The thermodynamic behavior of a steam turbine is influenced in use, for example, by erosion and corrosion, contamination (for example, by salt deposits), seal wear for example, on sealing strips), thermal deformation (for example, due to the maximum temperature limit being exceeded) and foreign body damage (for example, by impacts of welding beads on the blading).
It must be assumed that the aging phenomena listed above are always accompanied by an impairment in turbine efficiency and steam throughput during the operation of a steam turbine. Impairments in efficiency therefore not only equate to a lower utilization of the energy supplied to the steam turbine, but are also often an early indication of possible damage to steam turbine components around which steam flows. The same also applies to the steam throughput through a steam turbine. A deteriorating steam throughput under identical operating conditions, that is to say with an identical fresh steam pressure, identical inlet valve position and identical turbine rotational speed, likewise points to aging phenomena in the steam turbines.
The customary way of monitoring a steam turbine is to observe the operational indicators for conspicuous readings. This monitoring system has been refined by means of additional measurements of state variables, such as, for example, pressure and temperature at various points in the steam turbine. A further method for the early detection of aging phenomena on a steam turbine is to compare the current operating behavior with the theoretical operating behavior derived from the design of the steam turbine. The basis for this is mathematical models which are adopted from the design of the steam turbine plant and reproduce the thermodynamic behavior of the steam turbine.
Urban, L.A.: “Gas Path Analysis applied turbine engine condition monitoring”, AIAA Paper 72-1082, New Orleans, 1972, Fiedler, K., Lunderstädt, R.: “Diagnoseverfahren für RUSTON Gasturbine” [“Diagnostic Method for RUSTON Gas Turbines”], first part report, Gesellschaft für Forschung und Entwicklung mbH, Hamburg, 1985, and Lunderstäidt, R., Fiedler, K.: “Thernodynamische Zustandsdiagnose an Strömungsmaschinen” [“Thermodynamic Condition Diagnosis on Turbomachines”], yearbook 1992 of VDI Gesellschaft Energietechnik, VDI-Verlag, p. 160-178, Düsseldorf 1992, disclose a diagnostic method for aircraft turbine engines, in which state variables, such has the pressure and temperature of the gas turbine, are measured and diagnostic functions are calculated from these, it being possible to draw conclusions as to the aging of the gas turbine from the development in time of these diagnostic functions. The fluidic monitoring principle used there, which is known as gas path analysis, is based on a mathematical modeling of the flow processes in a gas turbine. The modeling principle forms the flowpath theory known in fluid mechanics.
This method has not been used for steam turbines, however, since the method was developed especially for gas turbines and gas turbines differ fundamentally in their form of construction from steam turbines.